Lift fresh or dried blood out of fabric with cold water and enzyme or peroxide treatments — never hot water.
Run cold water through the back of the stain so it pushes the blood out of the fibers rather than deeper in. Never use warm or hot water — heat cooks the proteins in blood and sets the stain permanently. The fresher the stain, the more cold water alone will remove.
Dab the stain with a clean cloth to lift blood away. Rubbing grinds it deeper into the weave and spreads it. Keep moving to a clean section of cloth as it picks up color.
For whites and colorfast fabrics, dab 3% hydrogen peroxide directly on the stain and let it bubble for a few minutes, then blot. For dried or set-in blood, work in an enzyme-based stain remover (or a liquid laundry detergent with enzymes) and let it sit 10–30 minutes. Test peroxide on a hidden seam first on colored fabric.
Launder in cold water. Before putting the item in the dryer, check that the stain is fully gone — the dryer's heat will set any remaining trace. If a shadow remains, repeat the treatment, because once heat-dried it is far harder to remove.